A conventional frequency converter, be it a single-phase or a three-phase circuit, comprises a rectifier bridge whose output is applied to an inverter. The inverter includes as many switching arms as there are phases to be fed, with each switching arm comprising a high rail transistor in series with a low rail transistor. Each transistor is associated with a regeneration diode. An intermediate point between the transistors is connected to one of the power windings of a motor.
A well-known problem in this type of converter lies in the fact that when both transistors in the same arm are simultaneously off while a current is flowing via one of the regeneration diodes, e.g. the high rail diode, the collector-emitter path of the high rail transistor is subjected to a negative voltage equal to the threshold voltage of the diode. Consequently, the base-collector junction of the transistor is forward biased and this gives rise to an accumulation of charge which must be evacuated in one way or another in order to avoid comprising proper operation of the inverter. The same problem exists with the low rail transistors when the low rail diodes are conducting.
Such a problem is easily solved with MOS type power transistors since charge is the majority carrier and it suffices merely to discharge the gate-source capacitance in which the charge accumulates. That is why this type of device is commonly used for switching in low power frequency converters.
However, when it is desired to work at higher powers, the high cost of MOS transistors becomes a major economic factor, and cheaper bipolar transistors are preferred. But then the above-mentioned problem applies in full: it is necessary to provide negative bias to the base of the transistor in order to evacuate the charge which tends to accumulate in the basecollector junction, with the charge now being constituted by minority carriers.
Any solution to the problem must take account of the fact that the emitters of the low rail transistors are referenced to a fixed potential whereas the emitters of the high rail transistors are referenced to a floating potential.
Known solutions for applying the desired negative bias are disadvantageous with respect both to cost and to bulk. In particular, the use of a transformer introduces stray capacitance which is prejudicial to proper operation of the converter, in particular at high switching frequencies.
The present invention seeks to avoid using a transformer and to mitigate the drawbacks of the prior art by proposing a switching circuit in which the required positive and negative bias is applied to the control electrode of a bipolar power transistor or of a power stage including such a transistor by means of an electronic circuit which is simple, compact, cheap, and capable of being integrated, and which has very low stray capacitance.